1. Field Of The Invention
This invention relates to secondary batteries and more particularly, to nonaqueous electrolyte secondary batteries which have a positive electrode containing a Li-containing metal complex oxide and a negative electrode made of a carbonaceous material.
2. Description Of The Prior Art
As is known in the art, various and novel types of portable electronic apparatus such as, for example, camera-assembled VTR, portable telephone sets, lap top-type microcomputers and the like, have been developed and put on the market one after another. There is now a great tendency toward compact and lightweight articles. Under these circumstances, batteries which are a portable mobile power source are required to be ones which have a higher energy density.
Main known secondary batteries are of the aqueous types which include, for example, lead batteries, nickel-cadmium batteries and the like. Although these batteries have good cycle characteristics, they are not satisfactory with respect to the energy density. In addition, the known batteries involve problems from the standpoint of ecology. Accordingly, there is a demand of developing novel types of secondary batteries in place of these conventional batteries.
Now, great attention has been directed to lithium nonaqueous electrolyte secondary batteries or so-called lithium rechargeable electrochemical cells which are free of any ecological problem and have a high energy density for high working potential.
In nonaqueous electrolyte batteries, the energy density of the battery depends on the characteristics of the positive electrode. Accordingly, there have been investigated and proposed a great number of active materials for the positive electrode.
In contrast, with secondary batteries, whether the development is successive or not depends on how a negative electrode composed of a lithium metal having good cycle characteristics can be developed. From this point of view, there remains little possibility of developing such a negative electrode of lithium.
For instance, a lithium secondary battery which is of the SUM-3 type and makes use of metallic lithium as the negative electrode has been reported with good characteristics. However, several troublesome problems involved in the lithium negative electrode have not been solved yet.
More particularly, With the case of nonaqueous electrolyte secondary batteries wherein lithium metal or alloys are employed as the negative electrode, when they are subjected to repetition of charge and discharge cycles, the metallic lithium is liable to electrodeposit as a powder, which is converted into dendrite-shaped crystals during the course of the charge cycle. As a consequence, they pass through fine holes of a separator film or interstices among fibers of a non-woven fabric separator to the positive electrode, whereupon internal short-circuiting takes place. Thus, a satisfactory charge and discharge cycle life is not ensured. In addition, since the metallic lithium is very active, there is a problem on safety.
To solve the problem, so-called Li-CIC (carbon-lithium intercalation compound) electrodes have been developed as a substitution for the lithium negative electrode and are considered to be promising with respect to the cycle life. More particularly, the Li-carbon intercalation compounds wherein lithium ions are intercalated in a kind of carbonaceous material are able to undergo reversible oxidation-reduction reaction in organic electrolytes containing lithium salts while accompanying electrochemical de-doping and doping of the lithium ions. The oxidation-reduction potential is in the range of about 0.02 to 1.0 volt, so that if combined with an appropriate material for positive electrode, such compounds will be usable as a material which is excellent as a negative electrode for non-aqueous electrolyte secondary batteries. In the discharge cycle of a battery system using the carbon-lithium intercalation compound as the negative electrode, the lithium ions doped in the carbon of the negative electrode are moved toward the positive electrode in which they play a role of escorting the electrons which are passed from the negative electrode through an outer circuit. On the other hand, in the charge cycle, the lithium ions which have been moved to the positive electrode are returned to the negative electrode wherein they play a role of escorting the electrons which are returned through the outer circuit. This means that in any stage of the charge and discharge cycles, any metallic lithium does not exist in the inside of the battery, thus not causing electrodeposition of inactive lithium or growth of dentrite as will occur in prior art counterparts. In addition, since active materials for the positive and negative electrodes are unlikely to suffer breakage of the crystal structure, very good charge and discharge cycle characteristics are ensured.
In the nonaqueous electrolyte secondary batteries, the characteristic properties of organic electrolytes used in the battery are very important for obtaining good charge and discharge characteristics. Many studies have been made on the relationship between the characteristics of organic electrolyte and the charge and discharge characteristics. With regard to the nonaqueous electrolyte second batteries using lithium as the negative electrode, the following knowledges are obtained.
1. The conductivity of organic electrolyte is remarkably improved using combinations of solvents with a high dielectric constant and solvents with a low viscosity. This can be semi-quantitatively explained in terms of the dissociation and mobility of ions in the electrolyte.
2. The higher conductivity of an electrolyte results in a smaller degree of polarization of the lithium negative electrode, with the tendency that the charge and discharge efficiencies become high.
3. Mixed systems of propylene carbonate, sulforane or diethylsulfoxide as the solvent with a high dielectric constant and 1,2-dimethoxyethane as the low viscosity solvent can yield high conductivity and good charge and discharge properties.
In this connection, however, we found as a result of intensive studies that when an electrolyte using a mixed solvent, for example, of propylene carbonate and 1,2-dimethoxyethane is employed, relatively good charge and discharge cycles are obtained at normal temperatures but when the battery is repeatedly charged and discharged at high temperatures, for example, of 40.degree. C., the capacity is abruptly lowered, so that the cycle life is inconveniently reduced to 1/10 of the life at normal temperatures.
As a matter of course, the secondary batteries to be used instead of existing Ni-Cd batteries and lead batteries have to be satisfactorily worked in a low to high temperature range of from at least -20.degree. C. to at least 45.degree. C. or over.
Accordingly, the abrupt lowering of the capacity under high temperature conditions in the nonaqueous electrolyte secondary battery using carbon-lithium intercalation compounds as the negative electrode greatly impedes the practical use of the battery.